1. Field of the Invention
The invention relates to a thermopile infrared sensor, thermopile infrared sensors array, and a method of manufacturing the same, in particular, to a thermopile infrared sensor having hidden thermocouple cantilever beams with low solid thermal conductance, and to a thermopile infrared sensors array having a high fill factor and a low noise equivalent temperature difference (NETD) and method of manufacturing the same.
2. Description of the Related Art
Thermocouples have been widely used for temperature measurement. By heating a junction between two conductors, a temperature difference between the junction and a portion located away from the junction is produced, thereby generating a diffusion current. A reverse electromotive force (also called the Seebeck voltage) is, therefore, corresponding to compensate the diffusion current. By measuring the Seebeck voltage, the temperature difference between the two ends of the thermocouple can be obtained. The value of the Seebeck voltage is determined from the product of the temperature difference between the two ends of the thermocouple and the Seebeck coefficient of the two conductors forming the thermocouple. Furthermore, the thermally generated voltage can be amplified by connecting plural pairs of thermocouples in series to form a thermopile. Therefore, the Seebeck voltage of the thermopile is equal to a value that is obtained from the product of the Seebeck voltage of a single thermocouple and the number of the thermocouples in series.
From 1980, with the development of silicon micromachining technologies, both the responsivity (V/W) and the response speed of the thermopile sensor is greatly increased by a suspending membrane structure that is capable of lowering its thermal capacitance and has a high thermal isolation effect (low thermal conductance). As a result, various high performance thermopile sensors are well developed.
In addition to a single sensor fabrication, it is also possible to manufacture a monolithic sensors array, mainly a infrared focal plane array (IRFPA) for thermal imaging in the fields of military detection systems, the automotive industry medicine, industrial automation and home security monitoring.
An advantage of a thermopile sensor is that it does not need a power supply. Thus, it rejects the noise voltage against the power source, which other thermal sensors such as a bolometer suffers from. Moreover, because the current flowing through the thermopile sensor is very small or even zero, a low frequency noise (1/f noise) caused by the driving current can also be ignored. Because thermopiles detect the temperature difference between the hot and the cold junctions, and because the cold junction is located on the heat reservoir, the cold junction plays the important role as the temperature reference. Therefore, the thermopile does not need temperature stabilization, whereas bolometers generally do. At the same time, the thermopile sensor does not need a chopper, whereas pyroelectric sensors do.
FIG. 1 is a locally enlarged schematic illustration showing a conventional thermopile infrared sensors array as disclosed by Kanno et al. in xe2x80x9cUncooled infrared focal plane array having 128xc3x97128 thermopile detector elements.xe2x80x9d It should be noted that only one complete thermopile sensor 10 is shown in FIG. 1.
Referring to FIG. 1, the thermopile sensor 10 includes a substrate 100 and a suspending membrane 101 that is formed on the substrate 100 and has a plurality of thermocouples 102. A hot junction 103 is located at the central portion of the suspending membrane 101, while a cold junction 104 is located on the peripheral portion of the suspending membrane 101 attached to the substrate 100 that acts as a heat reservoir. A plurality of etching windows 105 are formed on the suspending membrane 101. A polysilicon sacrificial layer (not shown) under the suspending membrane 101 can be etched via the etching windows 105 to construct a suspending structure. The structure of the thermopile sensor 10 will be better understood with reference to the cross-section view taken along a line 2xe2x80x942.
FIG. 2 is a cross-section view taken along the line 2xe2x80x942 as shown in FIG. 1. Referring to FIG. 2, the thermopile sensor 10 includes a substrate 100 and a suspending membrane 101. The substrate 100 includes an integrated circuit 107. Furthermore, a concavity 106 is formed between the suspending membrane 101 and the substrate 100. The suspending membrane 101 includes a first dielectric layer 108, a P-type polysilicon 102a, a second dielectric layer 109, an N-type polysilicon 102b, a third dielectric layer 110, and a metallic wiring 102c. The metallic wiring 102c connects the P-type polysilicon 102a with the N-type polysilicon 102b. A thermocouple 102 is composed of the P-type polysilicon 102a, the N-type polysilicon 102b, and the metallic wiring 102c. It should be noted that the regions of both the hot junction 103 and the cold junction 104 are also shown in this figure.
The thermopile infrared sensors array disclosed in Reference 1 is composed of 128xc3x97128 thermopile sensors 10 arranging in an array form, the specifications of which are as follows:
pixel size: (100xc3x97100) xcexcm2;
suspending membrane area: (80xc3x9784) xcexcm2;
fill factor: 67%
thermocouple: 32 pairs;
responsivity: 1550 V/W (in a vacuum environment); and
NETD (noise equivalent temperature difference): 0.5xc2x0 C.
The high performance of this cited example is due to the manufacturing processes of the sensors array are compatible with standard IC processing. Furthermore, the signals produced by these sensors are amplified through the corresponding integrated circuit 107. Therefore, a relatively high responsivity (1550 V/W) can be obtained. In addition, a sacrificial layer technology has been adopted to reach a high fill factor of 67%. Nevertheless, compared to the bolometric FPA (focal plane array) that has reported a low NETD of 0.039xc2x0 C., the sensitivity of this thermopile FPA is still not enough.
In order to improve the thermopile performance, the basic physics of this sensor should be analyzed first. FIG. 3 is a schematic illustration showing a simplified infrared imaging system, in which thermal radiation irradiated from an object 120 is absorbed by a thermopile sensor 122 via an optical system 121.
In an idealized situation, that is, assuming that there is no optical absorption by the delivery medium and the optical system 121, NETD is given by Equation (1):                     NETD        =                                            4              ⁢                              F                2                            ⁢              Vn                        RvAsL                    =                                                    4                ⁢                                  F                  2                                            AsL                        ⁢                          xe2x80x83                        ⁢            NEP                                              (        1        )            
wherein xe2x80x9cAsxe2x80x9d is the effective absorbing area of thermal radiation on the thermopile sensor 122; xe2x80x9cVnxe2x80x9d is the total noise voltage within the system bandwidth; xe2x80x9cRvxe2x80x9d is the responsivity; xe2x80x9cFxe2x80x9d is the focal ratio of the optical system 121; xe2x80x9cNEPxe2x80x9d is the noise equivalent power, the value of which is Vn/Rv; and xe2x80x9cLxe2x80x9d is the change in power per unit area radiated by the object 120 within a spectral band.
NETD which is defined as the change in temperature of the object 120 that will cause the signal-to-noise ratio at the output of the thermopile sensor 122 and its read-out electronic s to change by unity. Usually, NETD is used to judge the figure of merit of an infrared imaging system, and its value is lower to better
The responsivity Rv of the above thermopile sensor is represented by Equation (2):                               R          v                =                              η            ⁢                          xe2x80x83                        ⁢            N            ⁢                          xe2x80x83                        ⁢            α                                Gs            +            Gg            +            Gr                                              (        2        )            
wherein xe2x80x9cxcex7xe2x80x9d is the absorption of the incident radiation , xe2x80x9cNxe2x80x9d is the number of thermocouples connected in series, xe2x80x9cxcex1xe2x80x9d is the Seebeck coefficient (V/xc2x0 C.) of each thermocouple, and xe2x80x9cGsxe2x80x9d, xe2x80x9cGgxe2x80x9d, and xe2x80x9cGrxe2x80x9d are the solid, gas, and radiation thermal conductance, respectively, of the suspending membrane structure of the thermopile sensor.
For a thermopile, xe2x80x9cVnxe2x80x9d is mainly dominated by the Johnson noise; it is represented by Equation (3):
Vn={square root over (4+L kTsRxcex94f)}xe2x80x83xe2x80x83(3)
wherein xe2x80x9ckxe2x80x9d is the Boltzmann constant, xe2x80x9cTsxe2x80x9d is the temperature (K) of the thermopile sensor, xe2x80x9cRxe2x80x9d is the resistance of the thermocouple, and xe2x80x9cxcex94fxe2x80x9d is the system bandwidth.
From the viewpoint of sensor architecture, it is easily understood from the Equation (1) that a lower value of NEP should be the most desired result when the sensor area As is fixed. That implies either to increase the responsivity Rv or to reduce the noise voltage Vn, even to increase Rv and to reduce Vn simultaneously. Nevertheless, there is a strong relation between these two factors Rv and Vn. Because the total resistance R depends on each couple""s Seebeck coefficient xcex1 and number N, and is inversely dependent upon solid conductance Gs. That makes the situation rather sophisticated to resolve.
It is therefore an object of the invention to provide a low NETD thermopile infrared sensors array with a new sensor structure having hidden thermocouple cantilever beams, and a large fill factor. A method for manufacturing the sensor is also provided.
In accordance with one aspect of the invention, a thermopile infrared sensor includes a substrate, at least one thermocouple beam, and a suspending membrane. The thermocouple cantilever beam is formed above the substrate. The thermocouple cantilever beam has a first end and a second end located away from the first end. The first end is attached to the substrate to form a cold junction. A predetermined distance is formed between the second end and the substrate. The suspending membrane is formed above the thermocouple cantilever beam and is attached to the second end of the thermocouple cantilever beam to form a hot junction. The thermocouple cantilever beam is hidden underneath or covered by the suspending membrane.
In accordance with another aspect of the invention, a thermopile infrared sensors array is composed of a plurality of thermopile sensors each including a substrate, at least one thermocouple cantilever beam, and a suspending membrane. The thermocouple cantilever beam is formed above the substrate. The thermocouple cantilever beam has a first end and a second end located away from the first end. The first end attaches to the substrate to form a cold junction. A predetermined distance is formed between the second end and the substrate. The suspending membrane is formed above the thermocouple cantilever beam and is supported by the second end of the thermocouple cantilever beam to form a hot junction. The thermocouple cantilever beam is hidden under the suspending membrane.
The above thermopile sensor may further include an integrated circuit formed in the substrate and electrically connected with the at least one thermocouple cantilever beam to perform a predetermined processing to an output signal from the at least one thermocouple beam. In addition, the at least one thermocouple cantilever beam may include a thermocouple composed of an N-type and a P-type polysilicon conductor or composed of a polysilicon conductor and a metallic conductor.
In accordance with still another aspect of the invention, a method for manufacturing a thermopile infrared sensor comprising the steps of: forming at least one thermocouple and a first sacrificial layer on a substrate; forming a second sacrificial layer on both of the thermocouple and the first sacrificial layer, and a via hole on the second sacrificial layer to expose a part of the at least one thermocouple; forming a dielectric layer on both of the second sacrificial layer and the at least one thermocouple; removing periphery portions of the dielectric layer to expose periphery portions of the second sacrificial layer; etching the second sacrificial layer and the periphery portions of the first sacrificial layer to form undercuts under the dielectric layer; forming a black absorber on the dielectric layer by way of self-deposition; etching the second sacrificial layer and the first sacrificial layer through the undercuts to form a suspending membrane and expose a part of the substrate, wherein the suspending membrane is composed of the dielectric layer and the black absorber; and etching a predetermined portion of the substrate tinder the at least one thermocouple by way of anisotropic etching technology to form the at least one thermocouple beam in a cantilever form.